[llvm] [NFC] Move RemoveRedundantDbgInstrs outside of inner loop in JumpThreading (PR #123008)

[William Huang via llvm-commits]

https://github.com/huangjd created https://github.com/llvm/llvm-project/pull/123008

This cleanup action only needs to be performed once when the entire optimization is converged. Doing it in every iteration has a very high time-complexity, as it queries every dbg value in a dense map

Compare before and after for one internal source file with many basic blocks

![image](https://github.com/user-attachments/assets/1dac76a9-a974-4068-9aa1-4041f963fa8e)
![image](https://github.com/user-attachments/assets/73ea2ef1-d1f4-4064-8826-8c13fb539b8d)

>90% reduction in this extreme case.


>From 8aecbedfe3cd2df357007b498f861a3210b2685d Mon Sep 17 00:00:00 2001
From: William Huang <williamjhuang at google.com>
Date: Tue, 14 Jan 2025 22:46:02 -0500
Subject: [PATCH] [NFC] Move RemoveRedundantDbgInstrs outside of inner loop in
 JumpThreading

This cleanup action only needs to be performed once when the entire optimization
is converged. Doing it in every iteration has a very high
time-complexity.
---
 llvm/lib/Transforms/Scalar/JumpThreading.cpp | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 300a564e222e16..aae54d9763789b 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -330,11 +330,6 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
       while (processBlock(&BB)) // Thread all of the branches we can over BB.
         Changed = ChangedSinceLastAnalysisUpdate = true;
 
-      // Jump threading may have introduced redundant debug values into BB
-      // which should be removed.
-      if (Changed)
-        RemoveRedundantDbgInstrs(&BB);
-
       // Stop processing BB if it's the entry or is now deleted. The following
       // routines attempt to eliminate BB and locating a suitable replacement
       // for the entry is non-trivial.
@@ -366,7 +361,6 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
             // detect and transform nested loops later.
             !LoopHeaders.count(&BB) && !LoopHeaders.count(Succ) &&
             TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU.get())) {
-          RemoveRedundantDbgInstrs(Succ);
           // BB is valid for cleanup here because we passed in DTU. F remains
           // BB's parent until a DTU->getDomTree() event.
           LVI->eraseBlock(&BB);
@@ -377,6 +371,13 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
     EverChanged |= Changed;
   } while (Changed);
 
+  // Jump threading may have introduced redundant debug values into BB which
+  // should be removed.
+  if (EverChanged)
+    for (auto &BB : *F) {
+      RemoveRedundantDbgInstrs(&BB);
+    }
+
   LoopHeaders.clear();
   return EverChanged;
 }